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Test Horizontal Water Movement in Soil

To figure out the distance the water will move to the side (horizontally) in the soil you can perform a simple test. Perform this test some place in or near the area you plan to irrigate. Be aware that soil types can be different just a short distance apart. I've seen many rural home lots where the soil on one side of the property was sandy and it was clay on the other side. For the test you need to select an area of ground about 1.5 meters (5 feet) in diameter. It works best if the test area is undisturbed soil that has not been recently dug up or tilled. The top 150mm (6 inches) of soil in your test area needs to be dry, so allow it to dry out if it is wet. Clear the area of any surface debris and smooth the soil surface if it is uneven. Push a nail or some other marker into the center of your test area to identify it. (If rain prevents the test area from drying out, try building a low berm around the uphill side of the test area to keep rain water from draining onto it. Then cover the area with a tarp when it rains. Remove the tarp as soon as the rain stops so the sun and wind can help dry the soil.)

Start with an empty 2 liter plastic drink bottle or a gallon milk jug (the size of the container is not critical.) Heat a straight pin over a flame (hold the pin with pliers so you don't burn your fingers!) then use the pin to make a tiny pin hole in the bottom of the bottle. Put your finger over the hole in the bottle and fill the bottle with water. Now place the bottle in the middle of the test area and allow the water to flow out of the pin hole onto the soil next to the nail/marker you installed (it does not need to flow directly onto the marker, just in the general area around it.) The water will squirt out fast at first and then start to drip more slowly as the bottle empties. Once the bottle empties (it may take a couple of hours) refill it and repeat. You may refill the bottle a third time and repeat if you wish, but generally at this point more water does not make the wetted area any wider (the additional water just flows down deeper into the soil.) As the water flows from the bottle it is important that the water does not start to form a large puddle (more than 150mm or 6 inch diameter) on the soil surface. If it does start creating a large puddle remove the bottle for a while and allow some time for the water to soak into the soil. Then put the bottle back and allow more water to flow out onto the soil. Next wait at least 12 hours from the time you started the test to allow the water time to move into and through the soil. If your soil is clay you may need to wait as long as 3 days for the water to move the maximum distance. Once the wet area has stopped growing measure the size of the area that is moist. You should be able to visibly see a wet area around the marker, but since roots don't normally grow on the soil surface you should measure the distance below the surface. Dig three or four shallow trenches about 25mm to 50mm deep (1-2 inches) starting at the marker and extending in different directions toward the edge of the test area. Measure how far from the marker the soil is moist in the bottom of each trench. Don't be surprised if the wet area is larger below the soil surface, that is normal. Average the total lengths you measured to determine how far the water will soak sideways in your soil.

Make a pin hole in a 2 liter plastic bottle.

Fill the bottle with water and place in the middle of the test area. Refill after empty and repeat.

Allow the water to soak into the ground for 24 hours, then measure the radius of the area that is moist at a depth of 25mm.